On my second visit to Japan, I bothered to ask my friend and he said that they served the purpose of scaring off cats - introducing another form of Neko Yoke. Apparently, reflections in the water are supposed to generate holograms of fifteen feet tall fire breathing dragons which cats dont like.
Another theory (which sounds unlikely) is that the sun's reflections in the water scares cats. This Google Image search
will show you how folks use them to attempt to scare off cats, although after seeing this photo
, I'm not sure how effective they are ^^;